|Publication number||US7072531 B2|
|Application number||US 10/944,372|
|Publication date||Jul 4, 2006|
|Filing date||Sep 16, 2004|
|Priority date||Sep 16, 2004|
|Also published as||EP1831743A1, EP1831743A4, EP1831743B1, US20060056760, WO2006036242A1|
|Publication number||10944372, 944372, US 7072531 B2, US 7072531B2, US-B2-7072531, US7072531 B2, US7072531B2|
|Inventors||Kostadin D. Djordjev, Michael R. T. Tan, Chao-Kun Lin, Scott W. Corzine|
|Original Assignee||Agilent Technologies, Inc.|
|Export Citation||BiBTeX, EndNote, RefMan|
|Patent Citations (4), Referenced by (22), Classifications (9), Legal Events (8)|
|External Links: USPTO, USPTO Assignment, Espacenet|
The U.S. Government has a paid-up license in this invention and the right in limited circumstances to require the patent owner to license others on reasonable terms as provided for by the terms of contract No. MDA 972-03-3-004 awarded by the Defense Advanced Research Projects Agency (DARPA) of the U.S.
Communication systems based on modulated light sources are well known to the art. In high-speed communication systems, the light source is typically a laser. At frequencies below 10 GHz, the modulation can be imparted to the light source by turning the laser on and off. Unfortunately, this type of laser modulation leads to increased line width in the laser light. At frequencies at or above approximately 10 Ghz, this increased line width cannot be tolerated.
Accordingly, light sources that are to be modulated at frequencies above 10 GHz are typically constructed by providing a laser that runs continuously and a separate light modulator that modulates the intensity of the laser output. Modulators based on electro-absorption utilize a structure that is similar to a laser in that it includes a number of quantum well layers through which the light must propagate. The modulator typically has a transmissive state and an opaque state, which are switched back and forth by applying a potential across the modulator. The electrodes to which the signal is applied present a capacitive load to the driving circuitry, and hence, the modulator section is preferably as short as possible to minimize this capacitive load. In addition, high frequency driving circuitry preferably switches relatively small voltages, since such circuitry utilizes very small transistors that cannot withstand large voltages. Hence, low voltage, short modulators are preferred. Unfortunately, the length of the modulator must be sufficient to provide the desired contrast between the transmissive and opaque states of the modulator.
One promising design that provides short modulator sections that can operate at low voltage utilizes a resonant cavity that is coupled to a waveguide through which the signal that is to be modulated propagates. At “critical coupling”, the losses incurred by the light in making one trip around the resonator exactly equals the amount of light that is coupled into the resonator. When this occurs with light that has a wavelength equal to one of the resonances of the resonator, all of the light in the waveguide is extinguished, and hence the system has a transmission of 0. When the loss around the resonator is not at the critical coupling level, a portion of the light travels down the waveguide.
The amount of light absorbed in the resonator at each pass is determined by a voltage placed across the resonator. The voltage is set such that the resonator is critically coupled at a first voltage and less than critically coupled at a second voltage. Hence, by switching the voltage across the resonator between these two values, the light traveling in the waveguide is modulated from 0 to some transmission T that depends on the losses in the resonator at the second voltage. Ideally, T is equal to 1. That is, all of the light entering the waveguide leaves the waveguide in the transmissive state of the modulator. To achieve this ideal state, all of the losses in the resonator at the second voltage must be zero. This condition is difficult to meet in practical resonators, and hence, modulators of this design are less than ideal.
The present invention includes a light modulator having a waveguide and a resonator. The waveguide routes light of wavelength λ past the resonator. The resonator is coupled to the waveguide such that a portion of the light is input to the resonator, the resonator having a resonance at λ. The resonator includes a gain region in which light of wavelength λ is amplified and an absorption region in which light of wavelength λ is absorbed, the absorption region having first and second states, the first state absorbing less light of wavelength λ than the second state, the state of the absorption region is determined by an electrical signal coupled to the absorption region. The gain region provides a gain that compensates for the total light loss in the first state. In one embodiment, the waveguide and resonator are critically coupled when the absorption region is in the second state. The resonator can be of any geometry including a microdisk resonator, a micro-ring resonator, and a folded cavity resonator. In one embodiment, the resonator includes a layered structure having a quantum well layer, the quantum well layer having different bandgap energies in the gain and absorption regions. In one embodiment, the layered structure includes a waveguide region different from the gain and absorption regions in the layered structure, the quantum well layer in the waveguide region and the gain region-has different bandgap energies. In one embodiment, the resonator is vertically coupled to the waveguide.
FIGS. 5–-7 illustrate the construction of a modulator according to one embodiment of the present invention in the InP material system.
The manner in which the present invention operates may be more easily understood with reference to
The transmission past the resonator depends on two parameters, the power coupling factor into the resonator, and the losses inside the resonator. The losses in the resonator result from the absorption of the light by the material from which the resonator is constructed, scattering light lost at the bends in the waveguide, and light lost due to the change in absorption induced by altering the potential across the resonator. Refer now to
Ideally, the modulator is operated with one of its states at the critically coupled operating point. That is, one of the two absorption states described above is set to provide the attenuation needed for the resonator to be critically coupled in that state. The second state is chosen with two considerations in mind, the voltage needed to switch the absorber between the states and the transmission in the second state. Ideally, the second state would have an absorption of zero and correspond to a zero voltage across the absorber. This would correspond to operating between a zero loss point and the critically coupled point on the curves shown in
Unfortunately, achieving an absorption of zero is not possible with microdisk resonators constructed using economically practical fabrication systems. There are always some losses present even at zero voltage across the absorber. These losses arise from material losses, fabrication imperfections, surface roughness that scatters some of the light, etc. In this regard, it should be noted that even a small residual absorption in this region of the transmission curve leads to a large change in T. Hence, such devices have large losses even in the transmissive state. Furthermore, these losses will vary from device to device, and hence, the insertion loss may not be uniform from device to device.
Refer now to
Modulator 20 utilizes a resonator having an active gain section to compensate for the losses in resonator 22 incurred in the transmissive state, i.e., the losses incurred when the absorber section 23 is set to its minimum absorption. Since the residual losses are relatively small, gain section 24 need only provide a small gain to compensate for these losses. In modulator 22, the resonant cavity is divided into two sections that can be biased independent of one another. The bias voltage for absorption section 23 is provided by modulator controller 42, and the bias voltage for gain control section 24 is provided by gain controller 41. The bias voltage in section 23 is switched to modulate the light signal in the waveguide at point 16. The bias voltage in section 24 is maintained at a constant value to compensate for the losses in the resonator that are present when section 23 is set to the transmissive state. It is also desirable to have the gain section in the absorber so as to amplify only the resonant wavelength of interest.
The manner in which one embodiment of a modulator according to the present invention is fabricated will now be discussed in more detail. For the purposes of this discussion, it will be assumed that the modulator is in the same plane as the waveguide and that the modulator is a microdisk modulator as opposed to the ring modulator discussed above.
In one embodiment of the present invention the microdisk resonator and waveguide are constructed from InP-based materials. For the purposes of this discussion, any material that is lattice matched to InP within 2–5 percent will be deemed to be an InP-based material. For example, InGaAsP, AlInAs, AlInGaAs, InGaP, InGaAs, AlGaAsSb, AlAsSb are examples of such materials.
The modulator can be divided into three separate regions, the waveguide region, the absorption region of the resonator, and the active gain region of the resonator. All of these regions can be constructed by using a common set of waveguiding quantum well layers that provide a high-index of refraction and are sandwiched between low index of refraction p- and n-doped cladding layers. Denote the wavelength of the light to be modulated by λ. The various regions described above can be viewed as a set of common layers with different bandgap energies in the different regions. The waveguide is preferably transparent to light of wavelength λ. In addition, the absorption portion of the resonator is also preferably transparent to light of wavelength λ when no potential is applied across this portion of the resonator. This arrangement can be achieved by adjusting the bandgap in the quantum well region in the absorption section and the waveguide region such that the quantum well layer has an absorption peak 40–50 nm shorter than λ in the absorption region and 80–100 nm shorter than λ in the waveguiding region When the appropriate potential is applied to the absorption region, this absorption peak will shift to provide the needed absorption at λ. In the active region, the bandgap is set to provide gain to light of wavelength λ. Hence, these different bandgap regions can be created by starting with a layer having the bandgap needed by the active gain region. The bandgap in the waveguide and resonator areas is then lowered by impurity induced disordering or vacancy induced disordering. For example, the active layer can be masked to protect the active gain section from impurities implanted in the absorption region and waveguide region.
Refer now to
An InP sacrificial layer 132 is then deposited over the active layer, and the region that is to become the gain region is masked with an appropriate material such as SiN to protect the region from implantation. The surface of the stack of layers is then implanted with phosphorous ions in the region that is not protected by mask 131. The InP region protects the active region from the damage that it would incur if the implantation and masking were performed directly on the active region. The implanted stack of layers is then subjected to a rapid annealing at high temperature to allow the implanted ions to diffuse into the active region and alter the bandgap of the quantum well layers in that region. The InP protective layer and SiN mask are then removed by a wet etch that stops on the active region.
The above-described embodiments of the present invention require the creation of sub-micron features. To minimize bend losses, the microdisk resonator requires a large change in the index of refraction between the boundary of the resonator and the surrounding medium. This is accomplished by etching the area around the resonator and waveguide as described above. Unfortunately, this forces the width of the waveguide to be less than 0.5 μm. If the waveguide were wider than this, the waveguide would support multiple modes. While such structures are within the range of current manufacturing techniques, the cost of the submicron fabrication substantially increases the cost of the modulator. In addition, coupling into these narrow high index contrast waveguides from external optical fibers is difficult due to the different mode sizes. As a result, the coupling loss into the modulator increases.
Embodiments in which submicron structures are avoided can be constructed by using folded cavity resonators that do not require a large difference in index of refraction. Refer now to
Refer now to
The above-described embodiments of the present invention utilized specific resonator geometries, namely microdisks, micro-rings and folded cavities. However other geometries can be utilized. For example, resonators in the shape of a racetrack can be utilized. Any geometry that can accommodate both the absorption modulated region and the gain region can, in principle, be utilized.
The above-describe embodiments all utilized resonators that are in the same plane as the waveguide. However, arrangements in which the resonator is located over the waveguide and coupled vertically are also possible. Refer now to
Various modifications to the present invention will become apparent to those skilled in the art from the foregoing description and accompanying drawings. Accordingly, the present invention is to be limited solely by the scope of the following claims.
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|U.S. Classification||385/2, 359/247|
|International Classification||G02F1/17, G02F1/035|
|Cooperative Classification||G02B2006/12078, G02B6/12007, G02B2006/12142, G02B2006/12126|
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